The invention relates to a technique for forming a fragmented permeable mass of particles in an in situ oil shale retort. More particularly, this invention relates to technique for explosive expansion of unfragmented formation into voids excavated within the retort site which technique minimizes the occurrence of a relatively higher void fraction region along the side boundaries of the retort and a low void fraction region near the center of the retort.
The presence of large deposits of oil shale in the semi-arid high plateau region of the Western United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is, in fact, a misnomer, it is neither shale nor does it contain oil, it is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein and the liquid hydrocarbon product is called "shale oil."
A number of methods have been proposed for producing shale oil from oil shale; these generally involve either mining the kerogen-bearing shale and removing it to the surface for processing into shale oil or rubblization and processing of the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of large quantities of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents such as U.S. Pat. Nos. 3,661,423; 4,043,597; 4,043,598; and 4,153,298, as well as pending applications including U.S. patent application Ser. No. 929,250, filed July 31, 1978, by Thomas E. Ricketts, now U.S. Pat. No. 4,192,554, and U.S. patent application Ser. No. 070,319, filed Aug. 27, 1979, by Chang Yul Cha, entitled TWO-LEVEL HORIZONTAL FREE FACE MINING SYSTEM FOR IN SITU OIL SHALE RETORTS now abandoned. Each of these patents and applications is assigned to Occidental Oil Shale, Inc., assignee of this application, and each is incorporated herein by this reference.
These patents and applications describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein the formation is explosively expanded to form an in situ fragmented permeable mass of formation particles containing oil shale, referred to herein as a "retort" or as an "in situ oil shale retort". Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and that portion of the retort inlet mixture which does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale to a temperature sufficient to produce kerogen decomposition; this process, called "retorting," takes place in a retorting zone. Such decomposition of the oil shale in the retorting zone produces gaseous and liquid products, including gaseous and liquid hydrocarbons, and a residual carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. These products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn.
U.S. Pat. Nos. 4,043,597 and 4,043,598, and 4,192,554, disclose methods for explosively expanding formation containing oil shale toward horizontal free faces to form a fragmented mass in an in situ oil shale retort. According to such a method a plurality of vertically spaced apart voids of similar horizontal cross section are initially excavated one above another within the retort site. At least one zone of unfragmented formation is temporarily left between the voids. Explosive is placed in each of the unfragmented zones and detonated to explosively expand each unfragmented zone upwardly and/or downwardly towards the void or voids above and/or below it to form a fragmented mass having an average void volume about equal to the void volume of the initial voids. Retorting of the fragmented mass is then carried out as described above to recover shale oil from the oil shale.
U.S. Pat. No. 4,153,298 describes a method for forming a retort by excavating at least one horizontally extending void adjacent a zone of unfragmented formation to be expanded. At least one support pillar of unfragmented formation is left in the void for supporting overburden. Explosive is placed in the zone of unfragmented formation and in such a support pillar. Explosive in such a pillar and in the zone of unfragmented formation is detonated in a single round of explosions with a time delay between detonation of explosive in such a pillar and detonation of explosive in the zone of unfragmented formation for first expanding such a pillar toward the void and then expanding unfragmented formation toward the void. The time delay is sufficient for creation of a free face at the juncture of such a pillar and the zone of unfragmented formation. The time delay is short enough that explosive in the zone of unfragmented formation is detonated before particles formed by explosive expansion of the pillars have come to rest on the floor of the void.
Recovery of the shale oil resource is directly related to the distribution of the void fraction in the fragmented mass of oil shale particles. It is desirable to have a uniformly distributed void fraction in the fragmented mass so that there is generally uniform permeability, both horizontally across the retort and vertically along the length of the retort. With a uniformly distributed void fraction oxygen supplying gas and combustion gas can flow reasonably uniformly through the fragmented mass during retorting operations. A fragmented mass having generally uniform permeability prevents the retorting gas from bypassing portions of the fragmented mass as can occur if there is gas channelling through a portion of the mass due to non-uniform permeability.
It was found upon forming a retort generally in accordance with the description in U.S. Pat. No. 4,192,554 that the fragmented mass of particles had a relatively high void volume fraction region along the side boundaries of the retort and a low void fraction region nearer the center of the retort. It is theorized that during explosive expansion there is a tendency for oil shale to be expanded preferentially away from the walls of the retort due to the force balance of expanding gas generated by the explosion. Expanding gas from explosive in the central portion of the retort, according to this theory, encounters reasonably uniform resistance so that the net direction of expansion is essentially vertical. Near the side boundaries, however, gas pressure extending laterally toward the boundary is resisted by the unfragmented formation that will form the walls of the retort and the resultant force balance thus has a component directed away from the walls. This tends to cause preferential expansion away from the walls and results in a high void fraction region adjacent the walls. Another theory for understanding the high void fraction region near the walls assumes that particles in the central region of the retort can expand vertically without substantial rotation due to adjacent particles which are also expanding vertically. Particles adjacent the walls encounter the unfragmented walls and the resultant friction causes partial rotation. Such rotation, as contrasted with the non-rotation in the central region of the retort, can account for the relatively higher void fraction adjacent the walls of the retort. It is also possible that either or both of these phenomena may be occurring.